1. Field of the Invention
The invention relates in general to an audio control apparatus, and in particular, to an audio control apparatus to control an audio signal.
2. Description of the Related Art
Audio amplifiers exhibit clipping distortion during power amplification, resulting in noise from the speaker. To counter this problem, it is desirable to stop the power amplifiers from entering into the clipping distortion state, typically by limiting the amplitude of the input signal, or reducing the gain of the low power amplifiers.
FIG. 1 is a block diagram of a conventional amplitude limiter circuit. The audio control apparatus 10 in the conventional audio player system utilizes the voltage controlled amplifier 12 to limit signal amplitude. The gain from the voltage controlled amplifier 12 is modified by the control voltage Vtrl generated by the automatic level control unit 14. After amplifying the input audio signal, the amplified input audio signal is transmitted to the power amplifier 16 for amplification and then output by the speaker 18.
FIG. 2 is a block diagram of a voltage controller amplifier in a typical amplitude limiter. As indicated in FIG. 2, the voltage controlled amplifier (VCA) 12 basically comprises the differential amplifier 121 and the power supply 123. The voltage gain of the differential amplifier 121 is modified by changing the value of the current of the power supply 123. The automatic level control unit 14 is used for detecting the output amplitude of the voltage controlled amplifier 12. When the output amplitude exceeds a predetermined value, the automatic level control unit 14 decreases the gain of the voltage controlled amplifier 12, thereby limiting the output amplitude of the voltage controlled amplifier 12, preventing the power amplifier 16 from entering into the clipping distortion state.
However, the voltage controlled amplifier 12 has the following problems: due to the open loop operating condition, the distortion cannot be reduced, residual noise is high, and the dynamic range of the input signal and output signal are limited. Therefore, it is important to provide an audio control apparatus capable of limiting the amplitude of the input audio signal to solve the problems in conventional methods.